In the movie Blue Thunder, one of the plot devices used to build up
the mystique of Roy Scheider's character is his claim to have looped a
helicopter, which everyone else says is impossible. For years, I
believed looping a helicopter must be impossible (movies are all true,
aren't they?), but it's not. When I got involved in RC helicopters, I
was amazed to see model helicopters doing loops, rolls, inverted flight,
and all sorts of other "impossible" maneuvers.

For an example, see this video.
If you've never seen something like this before, you'll think you're
watching a special effects demo reel. Laws of physics are seemingly
violated, but it's all real. Full-size helicopters are capable of these
feats too, if they are rigged the right way. I suspect that when people
report seeing aircraft doing 90-degree turns over Area 51, they're not
seeing alien spacecraft: they're watching experimental helicopters.
Helicopter aerobatics make the most sophisticated airplane aerobatics
look boring.

Of course, I've wanted to do these maneuvers with my own RC helicopter.
The first helicopter I bought, the Ikarus Piccolo, was incapable of such
maneuvers. My current helicopter, the Hornet II, is capable. The
difference is that the Hornet features collective pitch control. The
thrust produced by a helicopter's rotor blades is dependent on two
things: the speed at which the rotor is turning, known as the head
speed and controlled by the throttle, and the angle of attack of the
blades with the air, known as the blade pitch. These throttle and
blade-pitch controls are known as the collective. As the head speed
increases, the lift produced by the blades increases, just like an
airplane's wing increases lift as its speed increases. Increasing the
angle of attack also increases lift. The Piccolo has fixed-pitch rotor
blades, meaning that the angle of attack never changes, so lift is
determined solely by the throttle. However, the Hornet can control both
the throttle and the pitch, providing for a greater degree of control
over lift. Collective pitch also provides quicker response, as changing
the pitch of blades can happen almost instantaneously, but speeding up
or slowing down the head speed takes some time.

To fly inverted (upside down), a helicopter pilot adjusts the blade
pitch so that it is opposite of normal blade pitch. This is called
negative pitch. Most real-world helicopters provide some degree of
negative pitch, which is needed for certain high-performance maneuvers
and for landing in high winds. However, real-world helicopters do not
provide enough negative pitch to hover or fly inverted. They could be
rigged for more negative pitch, but manufacturers do not want the
liability associated with certifying their helicopters for aerobatic
flight. In contrast, RC helicopters with collective pitch usually are
capable of inverted flight.

I first tried loops and rolls a few months ago, without much success. I
was able to roll successfully a few times, but I also crashed a few
times. My attempts to loop never worked out: I would end up with a messy
flip and recovery, or I would end up walking across the field to pick up
the pieces. The problem was that I didn't have enough cyclic power,
which is what controls left-right rolling and forward-backward pitch.
The helicopter just wouldn't turn over fast enough to get back to level
flight before hitting the ground.

I read of two potential fixes to this problem. The first fix was a
higher head speed. Higher head speeds increase the responsiveness of the
helicopter, but also make it more jumpy and difficult to control. The
second fix was addition of a Bell-Hiller mixer, which increases the
power of the cyclic, but also makes the helicopter more jumpy and
difficult to control. To smooth out the additional jumpiness, I added
weighted rotor blades, which stabilize the helicopter by increasing the
gyroscopic effect of the rotor.

I made these changes two months ago, but hadn't been able to try them
out. We had a few hurricanes blow through, which really screwed up our
weather for a few weeks. Then work ate up all my free time. Last week, I
finally tried it out. I was rusty, so I just did some simple hovering
and a few figure-eights. It was definitely a lot more responsive to the
controls, so much so that I had difficulty keeping it under control. i
did manage to keep it in the air for thirty minutes, and take it home
intact.

During the week, a made a few adjustments to my transmitter. I made the
cyclic control less sensitive and increased rudder sensitivity (I had
noticed difficulty getting the heli to turn sharply). I also did a lot
of practicing with the Reflex XTR simulator.

Today, I gave it another try. The air was calm when I packed up and got
in my car, but by the time I arrived at the flying site, there was a 10
MPH wind. Undeterred, I lifted off and did some easy flying for a while.
The wind was strong, but steady, so I figured I could try some crazy
stuff. I flew into the wind, went into a climb, applied negative pitch
as it went over the top of the loop, then leveled out. It wasn't pretty,
but it was a loop. I did a few more. None of them were pretty, and the
wind was too strong for me to know how they would look in steady wind,
but it was a lot easier than I expected. To tempt fate, I also did a
couple of rolls, and they were without incident.

So now I officially classify myself as an "intermediate" RC helicopter
pilot. Whenever we have a calm day, I'll try some inverted hovering,
which is what the Real Men do.

[UPDATE: Later in the day, I did my inverted hovering and a few more
loops and rolls. Unfortunately, I crashed during a simple right-side-up
hovering maneuver. I'm now waiting for $85 worth of replacement parts
before I can try again.]